This is a continuation of co-pending international application No. PCT/SE99/01720 filed on Sep. 29, 1999, which designated the United States of America.
Method of producing a paper having a three-dimensional pattern of alternating raised and recessed portions which is given the paper in connection with impulse drying, at which the wet paper web is passed through a press nip comprising a rotatable roll which is heated and that the paper web when passing through the press nip is given a three-dimensional pattern of alternating raised and recessed portions either by means of a pattern wire and/or by the fact that the heated roll is provided with a pattern intended to be pressed into the paper web against a holder-on.
Moist paper webs are usually dried against one or more heated rolls. A method which is commonly used for tissue paper is so called Yankee drying. At Yankee drying the moist paper web is pressed against a steam-heated Yankee cylinder, which can have a very large diameter. Further heat for drying is supplied by blowing of heated air. If the paper to be produced is soft paper the paper web is usually creped against the Yankee cylinder. The drying against the Yankee cylinder is preceded by a vacuum dewatering and a wet pressing, in which the water is mechanically pressed out of the paper web.
Another drying method is so called through-air-drying (TAD). In this method the paper is dried by means of hot air which is blown through the moist paper web, often without a preceding wet pressing. The paper web which enters the through-air-dryer is then only vacuum dewatered and has a dry content of about 25-30% and is dried in the through-air-dryer to a dry content of about 65-95%. The paper web is transferred to a special drying fabric and is passed over a so called TAD cylinder having an open structure. Hot air is blown through the paper web during its passage over the TAD cylinder. Paper produced in this way, mainly soft paper, becomes very soft and bulky. The method however is very energy-consuming since all water that is removed has to be evaporated. In connection with the TAD drying the pattern structure of the drying fabric is transferred to the paper web. This structure is essentially maintained also in wet condition of the paper, since it has been imparted to the wet paper web. A description of the TAD technique can be found in e.g. U.S. Pat. No. 3,301,746.
Impulse drying of a paper web is disclosed in e.g. SE-B-423 118 and shortly involves that the moist paper web is passed through the press nip between a press roll and a heated roll, which is heated to such a high temperature that a quick and strong steam generation occurs in the interface between the moist paper web and the heated roll. The heating of the roll is e.g. accomplished by gas burners or other heating devices, e.g. by means of electromagnetic induction. By the fact that the heat transfer to the paper mainly occurs in a press nip an extraordinarily high heat transfer speed is obtained. All water that is removed from the paper web during the impulse drying is not evaporated, but the steam on its way through the paper web carries along water from the pores between the fibers in the paper web. The drying efficiency becomes by this very high.
In EP-A-0 490 655 there is disclosed the production of a paper web, especially soft paper, where the paper simultaneously with impulse drying is given an embossed surface. This embossment is made by pressing a pattern into the paper from one or both sides against a hard holder-on. This gives a compression of the paper and by this a higher density in certain portions just opposite the impressions and a lower density in the intermediate portions.
In DE-A-26 15 889 there is disclosed a thermobonded embossed soft paper. Thermoplastic fibers are added to the paper web and after drying thereof the paper web is heated to a temperature exceeding the softening temperature of the thermoplastic fibers. Simultaneously with this heating the paper is pattern embossed. Through-air-drying is mentioned as a drying method.
The object of the present invention is to provide a method of producing an impulse dried paper having a three-dimensional pattern, e.g. a soft paper intended as toilet paper, kitchen rolls, paper handkerchiefs, table napkins and the like, and where the paper has a high bulk, high elasticity and a high softness. It is further a desire that the method should provide a controlled and broad pore volume distribution in the paper in order to optimize the absorption properties thereof. The paper structure should also essentially be maintained also in wet condition. This has according to the invention been provided by the fact that the wet paper web before entering said press nip is given a basis weight variation in a non-random pattern.
The basis weight variation that is given the paper during forming and dewatering is permanented in the following impulse drying step, at which the structure is essentially maintained also in wet condition of the paper.
According to a preferred embodiment of the invention the paper web is formed and/or dewatered on a wire whose dewatering capacity varies according to said non-random pattern and where the differences in dewatering capacity involves a certain movement of fibers and by that a local change of the basis weight of the paper web.
According to another embodiment the paper web is formed and/or dewatered in a converging forming space which on one side is delimited by a flat stationary or movable wall and on the other side by a wire having raised portions on the points where the wire threads are crossing each other, at which during dewatering in the converging forming space fibers are transferred from the raised portions of the wire cloth to intermediate portions resulting in a local change of the basis weight.
Further features and advantages of the invention are disclosed in the following description and in the dependant claims.